July 1, 2013

expert reaction to TB test-positive cattle entering the food chain

The Sunday Times broke a story about how cattle slaughtered after testing positive for bovine tuberculosis (bTB) were being sold for human consumption by Defra.

Professor Nigel Brown, President, Society for General Microbiology, said:

“TB is usually transmitted by direct contact with a living animal, or drinking TB-infected, unpasteurised milk. Catching TB from meat is highly unlikely, particularly as the primary infected organs, such as lungs and liver, are removed from the food chain. Cooking the meat to at least 60 degrees C will kill the TB bacterium, removing any chance of infection. I would be happy to eat meat from TB-infected cattle, and people should not be unduly concerned.

“One of the main reasons for TB testing of cattle is to eliminate TB from dairy herds, as it is possible to catch TB from infected, unpasteurised milk. Cows may be milked before their TB infection is known, and I strongly recommend that people do not drink raw milk.”

Professor Mark Fielder, General Secretary, Society for Applied Microbiology, said:

“We would not expect infective material from animals to enter the food chain. There are processes in place from farm to supermarket to ensure the safety of meat reaching our plates, including examination of carcasses by a vet.  So even if an animal with TB was slaughtered for food, it is extremely unlikely to be prepared for consumption by humans or other animals. If an animal with TB were to find its way into the food chain, regular good practice for handling and cooking meat ought to be sufficient to kill any bacteria still present.”

David Boxer, Chief Executive, Institute of Food Research, says:

“The Institute of Food Research has not conducted any research on the health implications of eating meat from cattle infected by bovine TB to date.  However we do know that all meat from cattle slaughtered due to bovine TB in the UK must undergo rigorous food safety checks before it can be passed fit for consumption.   Furthermore, to our knowledge, there are no confirmed known cases where TB has been transmitted through eating meat and the risk of infection from eating meat is extremely low.”

Professor Malcolm Bennett, Professor of Veterinary Pathology, University of Liverpool, said:

“The first thing to say is that this is not a new story and certainly not the discovery of any kind of cover-up or secret – the same story hits the headlines every few years, and the issue of whether or not there is any risk to public health has been looked at by expert groups not only in the UK but in Europe (EFSA). Cattle that test positive for TB are usually caught early during infection using a skin test that measures the immune response rather than a direct test for infection.  This is because it is very difficult to detect the bacteria until the animal becomes clinically ill, and because we want to diagnose and remove cattle from the population as early as possible and before they have the chance to infect other cattle. The farmer gets paid compensation by DEFRA, and DEFRA receives the price of the carcass at the abattoir in an attempt to offset some of the cost of compensation. The test-positive cattle are slaughtered at the end of the working day to minimise any risk of transmission or contamination, and like all carcasses, those of test-positive cattle are inspected for any signs of diseases that could stop the meat going off for human consumption: indeed these cattle are subject to extra scrutiny.  If there is no TB lesion, or only one site of TB lesions, then the affected organ is removed and the rest of the carcass goes off for human consumption – more than that (which would suggest spread of the disease) and the whole carcass is condemned.

“The bacteria don’t grow very well in muscle, so the risk of infection from traditional meat is lower still, and the bacteria are readily killed by cooking. The main risk is likely to be to people who handle infected/contaminated carcasses – and this is a well-recognised risk for American hunters handling venison, for example. Back when bovine TB was a huge public health problem (as it still is in some developing countries) the main source of human infection was drinking unpasteurised milk – the disease in humans of TB acquired by this route was called scrofula. The combination of controlling TB in cattle and, particularly, pasteurisation of milk has largely got rid of bovine TB in people in the UK: the few cases that are diagnosed each year are either imported cases or reactivations of infections from long ago or, occasionally, from drinking unpasteurised milk. 

“So the risk of getting TB from eating meat from these cattle is very low – and the FSA have stated that they know of no examples of this ever happening. I think that really this is an argument about risk and differing perceptions of what is acceptable and what is not.  In this case the argument has been muddied by the controversy over the control of bovine TB and in particular the role of badgers in the epidemiology of bovine TB. Currently bovine TB is not a significant public health issue in the UK, because we pasteurise most of our milk. Rather it is a problem of animal health and farming economics, of international trade agreements, of biosecurity and ecology, and of party politics – no less important for all that of course.  There is potential to damage public health should pasteurisation not work or the epidemic in cattle get even more out of hand than it currently is, but eating meat from reactor cattle is rather a diversion from more important aspects of the debate.”